A parasite infects the Doctor whilst the TARDIS is hovering in space,
and begins to slowly take over his mind. While the Doctor places
himself in a coma to stall the organism, Leela takes the Time Lord to a
medical facility on Titan in the far future. There, with the help of
Professor Marius and his robot dog K-9, Leela has miniaturised clones of
herself and the Doctor created, so that they can travel into the
Doctor's brain and take the battle to the parasite itself.

Production

The fifteenth season of Doctor Who began under a cloud. Graham
Williams had just taken over as producer from Philip Hinchcliffe, and
was immediately confronted with a directive from Head of Serials Bill
Slater to tone down the programme's levels of violence and horror. These
aspects had fostered controversy over the past several years, with
Season Fourteen's The Deadly Assassin
inviting particular criticism. Slater wanted Williams to take Doctor
Who in a more family-oriented direction; to this end, both Williams
and Tom Baker decided that humour should feature more prominently than
it had under Hinchcliffe.

Williams was also quickly faced with a major upheaval in his own vision
for Doctor Who. The new producer disliked the random nature of
the Doctor's adventures, and wanted to introduce some structure to the
series by having an entire season revolve around a recurring storyline.
Furthermore, Williams sought to portray the Doctor as a less anarchic,
more responsible figure. At first, Williams mooted a return to the UNIT
format which had been prevalent during Jon Pertwee's time as the Third
Doctor, but Slater vetoed this suggestion. By the end of November 1976,
Williams had instead developed the notion of the Doctor being despatched
on a quest through time and space for the six components of an
all-powerful “Key To Time”, acting on behalf of a
nigh-omnipotent figure called the White Guardian. Unfortunately,
Williams quickly discovered that there was no time to implement this
story arc for Season Fifteen, and decided to defer the idea until the
following year.

It was too late to implement Graham Williams' season-long
story arc involving the quest for the Key To Time

Instead, he and script editor Robert Holmes began to seek out more
traditional Doctor Who fare. On January 14th, 1977, four scripts
titled “Invisible Invader” were commissioned from Doctor
Who stalwarts Bob Baker and Dave Martin, whose most recent
contribution to the series had been the previous year's The Hand Of Fear. Their starting point was a
newspaper article about virus mutations, which suggested to the Bristol
Boys that an intelligent, adaptive virus could make an effective
antagonist. They married this notion to ideas inspired by the 1966
feature film Fantastic Voyage, in which scientists are reduced to
microscopic size and injected into the bloodstream of a dying
diplomat.

One notable feature of Baker and Martin's scripts was their decision to
use “Finglish” -- phonetic English -- throughout. For
instance, words like “Entrance” and “Exit” would
be spelt “Entruns” and “Egsit”, respectively. On
The Hand Of Fear, the Bristol Boys had
found the catchphrase “Eldrad must live” to be a popular
ingredient, and so they developed a similar mantra for “Invisible
Invader” in the form of the oft-repeated “Contact has been
made”. At one stage, once the Nucleus of the Swarm had entered the
macroscopic world, its baleful influence would have caused all of the
infectees to transform into similar monsters.

The most striking element of “Invisible Invader” was a
talking robot dog. This character was originally called FIDO which, in
the phonetic vernacular of the script, stood for “Phenomenological
Indication Data Observation unit”. Martin was a dog lover whose
own pet had recently been run over; FIDO was developed from the idea
that if only his dog had been “built like a tank”, the
animal would have survived the accident. Subsequently, the name Pluto
was considered (but quickly rejected, for fear of confusion with the
Walt Disney cartoon character of the same name), after which the
character was christened K-9.

Both Williams and Holmes were fond of K-9. Williams in particular
thought that the metal mutt would appeal tremendously to Doctor
Who's younger viewers. Thought was given to retaining K-9 beyond
“Invisible Invader”, and so considerable effort was spent on
developing the correct look for the robot dog, with two visual effects
designers -- Ian Scoones and Tony Harding -- submitting ideas. Scoones'
approach was for K-9 to be a large, armoured machine who could be
operated by a small actor. Williams discarded this notion because he
felt that K-9 should not be seen as simply a man in a costume. Harding's
initial attempt was very cartoonish; he was encouraged to make this more
streamlined and functional, while retaining the central concept that K-9
would be radio-controlled. Despite all of this work, however, Williams
was cautious about the feasibility of the character, and decided to
defer a final decision on K-9's future until he was able to judge how
well the prop worked in practise.

Graham Williams rejected one design which envisioned K-9
as a large, armoured machine operated by a small actor

Although “Invisible Invader” was intended to be the second
story broadcast as part of Season Fifteen, following Terrance Dicks'
“The Vampire Mutation”, it was planned to enter production
first. When Baker and Martin's scripts began to fall behind schedule,
however, the two serials briefly exchanged filming dates. During
February, Graeme McDonald (who had recently replaced Slater as Head of
Serials) became concerned that “The Vampire Mutation” might
undermine a serious version of Bram Stoker's Dracula in
preparation at the BBC for a Christmas 1977 broadcast. Consequently,
Dicks was forced to go back to the drawing board and began working on a
new adventure, Horror Of Fang Rock.
However, there was no way that this would be completed in time to make
the required production dates.

As a result, the story now called “The Invisible Invader”
was once again brought forward to the pole position of Doctor
Who's fifteenth recording block. Designated Serial 4T, it would be
directed by Derrick Goodwin. Goodwin's career in television had largely
centred on comedy shows (such as On The Buses and Thick As
Thieves), although he had also worked on an episode of Z
Cars. Goodwin later had to decline an invitation from Williams to
return to Doctor Who due to scheduling conflicts, so this turned
out to be his only work on the programme. As production loomed, other
titles for Serial 4T were considered, including “The Invader
Within” and “The Enemy Within”, before The
Invisible Enemy was selected.

With K-9 now a candidate for an ongoing place in Doctor Who, the
selection of an actor to voice the robot dog took on an increased
importance. Goodwin had recently renewed the acquaintance of John
Leeson, an actor he had first met while working in repertory theatre. On
television, Leeson had appeared in comedy and children's programmes,
including Dad's Army and Rainbow, and had also been a BBC
continuity announcer. On March 17th, he was contracted to voice K-9 for
the final three episodes of The Invisible Enemy. Eight days
later, his duties were extended to include the Voice of the Nucleus as
well. Also amongst Goodwin's cast was his wife, Elizabeth Norman,
playing Professor Marius' nurse.

Production of The Invisible Enemy began on March 22nd, when an
experimental effects session took place in Studio 6 of BBC Television
Centre. The serial's lavish modelwork was then filmed at Bray Studios
from March 28th to April 1st. This was to have been preceded by a week
of live-action filming at the Ealing Television Film Studios, but this
was cancelled in favour of an extra studio day. As such, The
Invisible Enemy was made in two three-day studio blocks, all in
TC6.

John Leeson took K-9's place during rehearsals, even
getting down on hands and knees beside his fellow performers

By now, Harding had finalised K-9's design, but the construction of the
prop's remote control mechanisms had to be contracted out to a firm
called the Radio Control Model Centre. Because the Doctor Who
budget did not allow for operator Nigel Brackley to be hired before the
actual recording dates, Leeson himself decided to take K-9's place
during rehearsals, even getting down on hands and knees beside his
fellow performers. One element of the K-9 prop that was very nearly
overlooked was the ticker tape which was emitted from the robot dog's
mouth: this omission was discovered at a very late stage, necessitating
the hasty enlargement of K-9's head to fit the device.

The first studio block took place from April 10th to 12th. The 10th and
11th were devoted to scenes in the shuttle, the refuel station, the
supervisor's office, and the mess room. April 12th, meanwhile,
concentrated on all of the material set within the Doctor's body. Two
weeks later, the second studio session spanned April 24th to 26th. The
first day included the debut of the new TARDIS console room. With the
wooden version introduced the previous year having warped in storage,
Williams opted to return to a simplified variation on the predominantly
white, more futuristic set employed during Doctor Who's first
thirteen seasons. This was conceived by longtime Doctor Who
designer Barry Newbery, who elected to simply retain the TARDIS console
prop which had been used prior to Season Fourteen.

The remainder of the second studio block involved sequences in the
Bi-Al Centre reception area, the isolation ward, the eye section, and
various corridors. The production found itself under increased pressure
compared to recent seasons, because one of Williams' instructions from
the BBC was to reign in Doctor Who's sometimes runaway budgets,
often resulting from lengthy shooting overruns. Unfortunately, the K-9
prop quickly proved itself to be a source of frustration in this regard
when it was discovered that the remote control mechanism sometimes
interfered with the cameras, resulting in visual distortion on the
recordings and the prop itself going haywire. Consequently, Tom Baker
quickly grew frustrated with K-9, occasionally going so far as to give
the prop a solid kick. Baker also disliked the fact that the robot dog's
short stature meant that he often had to stoop down so that they could
be in the same shot together. Fortunately, however, Baker and Leeson
quickly developed a strong camaraderie.

The script for The Invisible Enemy had been structured so that
the scene in which K-9 departs with the Doctor and Leela could simply be
dropped if the character was not to be retained, leaving the implication
that he had simply been returned to Professor Marius off-screen. Soon
after production on Serial 4T concluded, however, Williams decided to
keep K-9 on Doctor Who for the remainder of Season Fifteen,
despite the problems that the prop had posed. K-9 was revealed to the
press on June 10th. By this time, Tony Harding was already formulating
plans to overhaul the robot dog's inner workings, in the hope of one day
ensuring that K-9 would be no more disuptive a presence in the studio
than any other actor.